Field of the Invention
This invention relates to etching of polycrystalline silicon utilizing a plasma etch process.
Description of the Prior Art
Plasma etching of silicon and the production of semiconductor devices as well known and widely used at present. In the past, plasma etching of doped polycrystalline silicon has typically been accomplished by the use of various fluoro, fluorochloro, and fluoro-boro carbon gases such as CF.sub.4, CCl.sub.3 F, CF.sub.3 Br, and CCl.sub.2 F.sub.2. Gases of this type have the undesirable property that they etch isotropically and therefore cause lateral etching or undercut in addition to vertical etching. These materials also display a significant etch rate with regard to SiO.sub.2. More recently, the use of Cl.sub.2 etching at 100 kilohertz excitation frequency has been utilized as well as the use of hydrogen chloride, hydrogen bromide and helium as an etchant. Both of the latter etchants etch anisotropically in that they display negligible lateral etch rates or undercutting and also are highly selective to SiO.sub.2. As the device dimensions of semiconductor devices continues to shrink, there is a constantly greater demond on high etch rate ratios between two thin film materials. For example, it is required to etch polycrystalline silicon interconnects and stop on very thin dielectrics such as SiO.sub.2. Though highly selective etchants are available, as noted above, they often have the disadvantage of being unable to remove contaminants in the films to be etched. This often causes a residue of unetchable material on a wafer, possibly masking part of the film to be etched. These residues can cause problems in semiconductor manufacturing by blocking ion implants in further steps, causing high contact resistances, making inspections more difficult or by being a source of contamination that can cause delayed failure of the device. It is therefore desirable that these residues be completely removed during the etching process. In order to improve the reproducibility of the semiconductor devices, a selective etch using hydrogen chloride, hydrogen bromide and helium of the type set forth in Ser. No. 566,128 filed De. 27, 1983 was utilized in conjunction with a two-step etch. In the first step, a non-selective etch was utilized for the purpose of removing the very thin residue on the top of the polycrystalline silicon with the subsequent selective etch utilizing the etchant in the above copending application. The initial non-selective etch was for about 10 Angstroms since a layer of SiO.sub.2 of about this thickness normally forms on the surface of exposed silicon and the second etch step went the rest of the way in an anisotropic manner. This process still did not provide desirable results and residues were still left at the bottom of the etch. This is apparently due to the fact, as has now been determined, that silicon dioxide forms in the interstices extending to the surface of the polycrystalline silicon and extends below the surface line and between the crystal grains, a distance greater than 10 Angstroms. Also, other impurities may be involved.